U.S. patent number 8,267,321 [Application Number 12/366,857] was granted by the patent office on 2012-09-18 for enhanced-density barcode.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Michael Peter Kuyper-Hammond, Michael Charles Osborne, Tamas Visegrady.
United States Patent |
8,267,321 |
Kuyper-Hammond , et
al. |
September 18, 2012 |
Enhanced-density barcode
Abstract
A method of decoding a two-dimensional enhanced-density barcode.
A first and a second barcode are encoded in the enhanced-density
barcode. The enhanced-density barcode includes a set of blocks.
Each block includes a predefined number of sub-pixels. The blocks
of the enhanced-density barcode being arranged relatively to each
other in a geometrical lattice having a first and a second lattice
direction. The method includes the steps of distorting of the
enhanced-density barcode in the first lattice direction, resulting
in a first distorted barcode, distorting of the enhanced-density
barcode in the second lattice direction, resulting in a second
distorted barcode, reconstructing the first barcode by low-pass
filtering the first distorted barcode, reconstructing the second
barcode by low-pass filtering the second distorted barcode.
Inventors: |
Kuyper-Hammond; Michael Peter
(Kaltbrunn, CH), Osborne; Michael Charles
(Richterswil, CH), Visegrady; Tamas (Zurich,
CH) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
40938064 |
Appl.
No.: |
12/366,857 |
Filed: |
February 6, 2009 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
|
US 20090200379 A1 |
Aug 13, 2009 |
|
Foreign Application Priority Data
|
|
|
|
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Feb 11, 2008 [EP] |
|
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08101467 |
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Current U.S.
Class: |
235/462.09;
235/494; 235/462.1; 235/462.04; 235/456; 235/462.25 |
Current CPC
Class: |
G06K
19/06028 (20130101) |
Current International
Class: |
G06K
7/10 (20060101) |
Field of
Search: |
;235/462.1,462.04,462.09,462.25,494,456 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lee; Michael G
Assistant Examiner: Chedekel; Tabitha
Attorney, Agent or Firm: Zarick; Gail H. Otterstedt,
Ellenbogen & Kammer, LLP
Claims
What is claimed is:
1. A method comprising the steps of: accessing a two-dimensional
enhanced-density barcode to be decoded, wherein a first
two-dimensional barcode and a second two-dimensional barcode are
combined by overlaying and encoded in said two-dimensional
enhanced-density barcode, the two-dimensional enhanced-density
barcode including a set of blocks, each block including a
predefined number of sub-pixels, the blocks of the two-dimensional
enhanced-density barcode being arranged relative to each other in a
geometrical lattice, and the lattice having a first and a second
lattice direction; distorting the two-dimensional enhanced-density
barcode in the first lattice direction, resulting in a first
distorted two-dimensional barcode; distorting the two-dimensional
enhanced-density barcode in the second lattice direction, resulting
in a second two-dimensional distorted barcode; reconstructing the
first two-dimensional barcode by low-pass filtering the first
two-dimensional distorted barcode; and reconstructing the second
two-dimensional barcode by low-pass filtering the second
two-dimensional distorted barcode thereby producing two decoded
two-dimensional barcodes corresponding to said first
two-dimensional barcode and said second two-dimensional barcode
encoded in said two-dimensional enhanced-density barcode.
2. The method according to claim 1, wherein: the distorting of the
two-dimensional enhanced-density barcode in the first lattice
direction results in a distortion of a first group of said
sub-pixels in the first lattice direction, the distortion of the
first group of said sub-pixels in the first lattice direction
extending from respective sub-pixels of the first group of said
sub-pixels to adjacent ones of said sub-pixels in the first lattice
direction; and the distorting of the two-dimensional
enhanced-density barcode in the second lattice direction results in
a distortion of the first group of said sub-pixels in the second
lattice direction, the distortion of the first group of said
sub-pixels in the second lattice direction extending from the
respective sub-pixels of the first group of said sub-pixels to
adjacent ones of said sub-pixels in the second lattice
direction.
3. The method according to claim 2, wherein the first group of said
sub-pixels consist of sub-pixels of a first predefined colour.
4. The method according to claim 1, wherein the first and the
second lattice directions are perpendicular to each other.
5. The method according to claim 2, wherein: the distorting in the
first lattice direction results in a smearing of the sub-pixels of
the first group of sub-pixels in the first lattice direction; and
the distorting in the second lattice direction results in a
smearing of the sub-pixels of the first group of sub-pixels in the
second lattice direction.
6. The method according to claim 1, wherein the distorting of the
two-dimensional enhanced-density barcode is performed by
anisotropic filtering.
7. The method of claim 1, wherein the distorting and the low-pass
filtering are performed by optical or digital means.
8. The method of claim 1, wherein: the first two-dimensional
barcode has a barcode axis corresponding to a barcode axis of the
second two-dimensional barcode; and the first and the second
lattice directions correspond to the barcode axes of the first and
the second two-dimensional barcodes.
9. The method of claim 1, further comprising the step of:
reconstructing the first and the second lattice directions.
10. The method of claim 9, wherein the step of reconstructing the
first and the second lattice directions is performed by analyzing
alignment patterns included in the two-dimensional enhanced-density
barcode.
11. The method of claim 10, wherein the alignment patterns are
given by additional predefined bar arrangements or additional grid
lines included in the two-dimensional enhanced-density barcode.
12. The method of claim 11, wherein the low-pass filtering of the
first and the second distorted two-dimensional barcodes is adapted
to colour-average the sub-pixels of the blocks to pixels of the
first and the second decoded two-dimensional barcodes
respectively.
13. The method of claim 12, wherein the steps of reconstructing the
first and the second barcodes includes determining block boundaries
of the blocks.
14. The method of claim 12, wherein the steps of reconstructing the
first and the second barcodes are adapted to interpret a colour
value below a predetermined threshold as a first block colour and
above the predetermined threshold as a second block colour.
15. The method of claim 13, wherein the block boundaries are
determined by analyzing the alignment patterns.
16. The method of claim 1, wherein the sub-pixel density of the
enhanced-density two-dimensional barcode is higher than the pixel
density of the first and the second two-dimensional barcodes.
17. A barcode reader comprising: means for accessing a
two-dimensional enhanced-density barcode to be decoded, wherein a
first two-dimensional barcode and a second two-dimensional barcode
are combined by overlaying and encoded in the two-dimensional
enhanced-density barcode, the two-dimensional enhanced-density
barcode including a set of blocks, each block including a
predefined number of sub-pixels, the blocks of the two-dimensional
enhanced-density barcode being arranged relative to each other in a
geometrical lattice, the lattice having a first and a second
lattice direction; means for distorting the two-dimensional
enhanced-density barcode in the first lattice direction, resulting
in a first distorted two-dimensional barcode; means for distorting
the two-dimensional enhanced-density barcode in the second lattice
direction, resulting in a second two-dimensional distorted barcode;
means for reconstructing the first two-dimensional barcode by
low-pass filtering the first two-dimensional distorted barcode; and
means for reconstructing the second two-dimensional barcode by
low-pass filtering the second two-dimensional distorted barcode
thereby producing two decoded two-dimensional barcodes
corresponding to said first two-dimensional barcode and said second
two-dimensional barcode encoded in said two-dimensional
enhanced-density barcode.
18. The reader according to claim 17, wherein the means for
distorting and the means for low-pass filtering are optical or
digital means.
19. The reader according to claim 17, further comprising: means for
reconstructing the first and the second lattice directions.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority, under 35 U.S.C. .sctn.119, from
European Patent Application No. 08101467.2 filed Feb. 11, 2008, the
entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of decoding a
two-dimensional enhanced-density barcode, a method of encoding a
two-dimensional enhanced-density barcode, a barcode reader for
reading a two-dimensional enhanced-density barcode and a
two-dimensional enhanced-density barcode.
2. Description of Related Art
Two-dimensional (2D) barcodes, widely used to encode binary
information on surfaces, have information density limits due to
resolution limitations of reading equipments. For applications
where the available encoding surface may be scaled according to the
data requirements this does not cause a problem. However, for
applications that have a limited encoding space this restricts the
amount of usable data that may be stored. One such example is a
standard ID-1 form driving license according to the ISO/IEC
standard 18013-1 of the International Standard Organization (ISO)
that is required to carry a 2D barcode containing an image of the
cardholder. The space available for encoding a 2D barcode is
limited which restricts the size of the image that can be stored
which means that only low resolution images can be used.
SUMMARY OF THE INVENTION
The present invention is directed to a method for decoding, a
method for encoding, a barcode reader and an enhanced-density
barcode.
According to a first aspect of the invention there is provided a
method of decoding a two-dimensional enhanced-density barcode,
wherein a first and a second barcode are encoded in the
enhanced-density barcode. The enhanced-density barcode includes a
set of blocks, each block including a predefined number of
sub-pixels. The blocks of the enhanced-density barcode are arranged
relative to each other in a geometrical lattice, which has a first
and a second lattice direction. The method includes the steps of:
distorting the enhanced-density barcode in the first lattice
direction, resulting in a first distorted barcode; distorting the
enhanced-density barcode in the second lattice direction, resulting
in a second distorted barcode; reconstructing the first barcode by
low-pass filtering the first distorted barcode; and reconstructing
the second barcode by low-pass filtering the second distorted
barcode.
According to a second aspect of the invention there is provided a
barcode reader for reading a first and a second barcode from a
two-dimensional enhanced-density barcode, wherein the first and the
second barcode are encoded in the two-dimensional enhanced-density
barcode, the enhanced-density barcode including a set of blocks,
each block including a predefined number of sub-pixels, the blocks
of the enhanced-density barcode being arranged relatively to each
other in a geometrical lattice, the lattice having a first and a
second lattice direction, the reader including a component for
distorting the enhanced-density barcode in the first lattice
direction, resulting in a first distorted barcode, a component for
distorting the enhanced-density barcode in the second lattice
direction, resulting in a second distorted barcode, a component for
reconstructing the first barcode by low-pass filtering the first
distorted barcode, a component for reconstructing the second
barcode by low-pass filtering the second distorted barcode.
According to a third aspect of the invention there is provided
method of encoding a two-dimensional enhanced-density barcode, the
method including the steps of receiving a first and a second
barcode, the first and the second barcode including pixels of a
predefined pixel density, performing a pair wise encoding of pixel
pairs of the first and the second barcode according to a predefined
encoding scheme, wherein the predefined encoding scheme allocates
to each combination of pixel pairs a predefined block including a
predefined number of sub-pixels, wherein the blocks of the
enhanced-density barcode are arranged relatively to each other in a
geometrical lattice, the lattice having a first and a second
lattice direction.
According to a fourth aspect of the invention there is provided
two-dimensional enhanced-density barcode, wherein a first and a
second barcode are encoded in the two-dimensional enhanced-density
barcode by means of a predefined encoding scheme, wherein according
to the predefined encoding scheme each combination of pixel pairs
of the first and the second barcode is mapped on a predefined block
including a predefined number of sub-pixels, wherein the blocks of
the enhanced-density barcode are arranged relatively to each other
in a geometrical lattice, the lattice having a first and a second
lattice direction.
Any of the features of one aspect of the invention may apply to the
other aspects of the invention. Advantages of the features of one
aspect of the invention may apply to the other aspects of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The following preferred embodiments of the invention will be
described in detail by way of example only making reference to the
drawings in which:
FIG. 1: is a block diagram of a barcode reader;
FIG. 2: is a block diagram of a barcode writer;
FIG. 3: is a schematic illustrating an embodiment of the encoding
and decoding process according to the invention;
FIG. 4: is a schematic illustrating a symbol set used to perform an
encoding process;
FIG. 5: is a schematic illustrating a smearing process of various
symbols; and
FIG. 6: is a schematic illustrating a block encoding and effects of
smearing.
DETAILED DESCRIPTION
According to a first aspect of the invention there is provided a
method of decoding a two-dimensional enhanced-density barcode,
wherein a first and a second barcode are encoded in the
enhanced-density barcode, the enhanced-density barcode including a
set of blocks, each block including a predefined number of
sub-pixels, the blocks of the enhanced-density barcode being
arranged relatively to each other in a geometrical lattice, the
lattice having a first and a second lattice direction, the method
including the steps of distorting of the enhanced-density barcode
in the first lattice direction, resulting in a first distorted
barcode, distorting of the enhanced-density barcode in the second
lattice direction, resulting in a second distorted barcode,
reconstructing the first barcode by low-pass filtering the first
distorted barcode, and reconstructing the second barcode by
low-pass filtering the second distorted barcode.
The method according to this aspect of the invention has the
advantage that the effective data rate of a barcode can be
increased by combining several low resolution images into a single,
high resolution barcode. Individual components can be reconstructed
from the combined image using distortion and low pass filtering
steps such that, after applying the distortion and the filtering
steps, scanning equipment can be used to read the first and the
second barcode from the enhanced-density barcode. High resolution
digital processing can be used to combine the first and the second
lower resolution barcodes. An encoded output may consist of low
frequency areas where constituent barcode pixels are identical and
high frequency regions where the corresponding pixels differ. High
frequency pixel encoding may rely on features finer than the
effective resolution of the scanning equipment. Nevertheless, by
performing the distortion and filtering steps according to the
embodiments of the invention, low resolution scanning equipment can
be used to decode and extract the first and the second barcode
encoded in the enhanced-density barcode.
The terms blocks and sub-pixels have to be broadly understood as
general terms covering all kinds of geometrical shapes like
rectangles, squares, circles, polygons etc.
An enhanced-density barcode is understood as a barcode that has a
higher density than the first and the second barcode. In other
words, the enhanced-density barcode can store or represent more
information than the single first and second barcode.
According to an embodiment of the invention the enhanced-density
barcode can store the information of the first and the second
barcode together on an area sufficient to store the larger of
them.
A two-dimensional barcode is a barcode in which data is encoded in
two dimensions, e.g. a horizontal and vertical dimension.
According to an embodiment of the first aspect of the invention the
distorting of the enhanced-density barcode in the first lattice
direction results in a distortion of a first group of sub-pixels in
the first lattice direction, the distortion of the first group of
sub-pixels in the first lattice direction extending from the
respective sub-pixels of the first group to the sub-pixels adjacent
in the first lattice direction. Furthermore, the distorting of the
enhanced-density barcode in the second lattice direction results in
a distortion of the first group of sub-pixels in the second lattice
direction, the distortion of the first group of sub-pixels in the
second lattice direction extending from the respective sub-pixels
of the first group to the sub-pixels adjacent in the second lattice
direction.
According to an embodiment of the first aspect of the invention the
first group of sub-pixels consists of sub-pixels of a first
predefined colour. The predefined colour is preferably a dark
colour, in particular black. Such a colour can be distorted in an
efficient way. The second group of sub-pixels has a different
colour than the first group of sub-pixels, preferably a light
colour such as white. Such a colour will not be distorted or
substantially not distorted.
In accordance with an embodiment of the invention, the first and
the second lattice direction are perpendicular to each other.
However, in general the first and the second lattice direction can
have any arbitrary orientation relative to each other.
According to an embodiment of the invention the distorting in the
first lattice direction results in a smearing of the sub-pixels of
the first group of sub-pixels in the first lattice direction and
the distorting in the second lattice direction results in a
smearing of the sub-pixels of the first group of sub-pixels in the
second lattice direction.
Such a smearing performs an anisotropic transformation of the
enhanced-density barcode. The smearing process distorts sub-pixels
of the first group of sub-pixels, in particular dark sub-pixels,
thereby creating a lighter, but observable "shadow" that darkens
neighboring light pixels in the direction of smearing. Such
smearing does not lighten sub-pixels and only dark sub-pixels of
the first group of sub-pixels influence and spread over to adjacent
light sub-pixels. In other words, dark sub-pixels of the first
group of sub-pixels generate an observable shadow. Preferably the
shadow, i.e. the distorting influence, does not exceed a sub-pixel
distance. According to an embodiment of the invention the shadow
has substantially the width of one sub-pixel.
In accordance with a further embodiment of the invention, the
distorting is performed by anisotropic filtering. In general, the
distorting and/or the low pass filtering are performed optically or
digitally. However, preferably the distorting and the low pass
filtering are performed optically since in this case low resolution
scanning equipment can be used which just reads the decoded
analogue optically decoded first and second barcode.
In accordance with an embodiment of the invention, the barcode axis
of the first barcode corresponds to the barcode axis of the second
barcode, wherein the first and the second lattice direction
correspond to the barcode axis of the first and the second
barcode.
In accordance with an embodiment of the invention, the method
further includes reconstructing the first and the second lattice
direction. For example, the reconstruction is performed by
analyzing special alignment patterns included in the
enhanced-density barcode. Such alignment patterns may be given by
additional predefined sub-pixel arrangements and/or additional grid
lines included in the enhanced-density barcode. This simplifies the
locating of the first and the second lattice direction and prevents
a wrong reconstruction of the first and second lattice direction.
Furthermore, this reduces the error proneness.
According to an embodiment of the invention the low-pass filtering
of the first and the second distorted barcode is adapted to
colour-average the sub-pixels of the blocks to pixels of the first
and the second barcode respectively.
Preferably, reconstructing the first and the second barcode further
includes determining the block boundaries of the blocks. Once block
boundaries have been determined, the low pass filtering is adapted
such to transform the block contents by color-averaging to, for
example, a single black or a single white pixel value, which can
then be used as a single pixel of the first or the second barcode.
Low pass filtering may be implicit if the scanning equipment does
not have sufficient resolution to oversample the image. In this
case, pixels of the first and the second barcode are reconstructed
from a single reading combining the luminosity of an entire block.
However, with sufficient high resolution scanning equipment,
several readings from the same block will be averaged to a single
binary value. In both cases, the sub-pixels included in a block
will be reduced to a single pixel reading.
According to an embodiment of the invention the reconstructing is
adapted to interpret a colour value below a predetermined threshold
as a first block colour and above the predetermined threshold as a
second block colour.
In case of a black and white barcode, a color value above the
threshold would be interpreted as black block color and hence
reconstructed to a black pixel of the first or the second barcode
respectively. A color value below the threshold would be
interpreted as a white block color and hence reconstructed to a
white pixel of the first or the second barcode respectively.
As an example, the threshold could be defined by 55% black/dark
sub-pixels. Accordingly, if a block of four sub-pixels comprises 3
or 4 black/dark sub-pixels, it would be color-averaged to a
black/dark pixel. If a block of four sub-pixels comprises 2, 1 or
zero black/dark sub-pixels (corresponding to 2, 3 or 4 white/light
sub-pixels), it would be color-averaged to a white/light pixel.
According to an embodiment of the invention the predetermined
threshold is understood as predetermined darkness, e.g. a darkness
of 55%. The exact thresholds may be selected based on knowledge of
the distortion or smearing process.
In accordance with an embodiment of the invention, the block
boundaries are determined by analyzing the alignment patterns. By
using the alignment patterns to determine the block boundaries,
blocks can be accurately determined which may further reduce the
error proneness. Furthermore, special analysis and correction
algorithms to determine block boundaries can be avoided which
simplifies the technical design of scanning equipment.
In accordance with an embodiment of the invention the sub-pixel
density of the enhanced-density barcode is higher than the pixel
density of the first and/or the second bar code. This has the
advantage that within a given spatial area more information can be
encoded.
According to a second aspect of the invention there is provided a
barcode reader for reading a first and a second barcode from a
two-dimensional enhanced-density barcode, wherein the first and the
second barcode are encoded in the two-dimensional enhanced-density
barcode, the enhanced-density barcode including a set of blocks,
each block including a predefined number of sub-pixels, the blocks
of the enhanced-density barcode being arranged relatively to each
other in a geometrical lattice, the lattice having a first and a
second lattice direction, the reader including a component for
distorting the enhanced-density barcode in the first lattice
direction, resulting in a first distorted barcode, a component for
distorting the enhanced-density barcode in the second lattice
direction, resulting in a second distorted barcode, a component for
reconstructing the first barcode by low-pass filtering the first
distorted barcode, a component for reconstructing the second
barcode by low-pass filtering the second distorted barcode.
The components for distorting and low-pass filtering are preferably
optical components and/or digital components.
According to an embodiment of the invention the barcode reader
includes a component for reconstructing the first and the second
lattice direction.
According to a third aspect of the invention there is provided
method of encoding a two-dimensional enhanced-density barcode, the
method including the steps of receiving a first and a second
barcode, the first and the second barcode including pixels of a
predefined pixel density, performing a pair wise encoding of pixel
pairs of the first and the second barcode according to a predefined
encoding scheme, wherein the predefined encoding scheme allocates
to each combination of pixel pairs a predefined block including a
predefined number of sub-pixels, wherein the blocks of the
enhanced-density barcode are arranged relatively to each other in a
geometrical lattice, the lattice having a first and a second
lattice direction.
According to a fourth aspect of the invention there is provided
two-dimensional enhanced-density barcode, wherein a first and a
second barcode are encoded in the two-dimensional enhanced-density
barcode by means of a predefined encoding scheme, wherein according
to the predefined encoding scheme each combination of pixel pairs
of the first and the second barcode is mapped on a predefined block
including a predefined number of sub-pixels, wherein the blocks of
the enhanced-density barcode are arranged relatively to each other
in a geometrical lattice, the lattice having a first and a second
lattice direction.
According to an embodiment of this aspect of the invention the
sub-pixel density of the enhanced-density barcode is four times
higher than the pixel density of the first and the second barcode.
In other words, each pixel pair established by a pixel of the first
barcode and a pixel of the second barcode is mapped on a block
consisting of four sub-pixels. Each block of the enhanced-density
barcode has the size of one single pixel of the first barcode and
the second barcode respectively.
FIG. 1 is a block diagram of a barcode reader. The barcode reader
includes a scanner 102 and a data processing system 100. The
barcode reader further includes a component for low pass filtering
and a component for distorting an enhanced-density barcode 104. The
data processing system 100 includes a processor 110 and a memory
112, such as a hard disc or a random access memory (RAM). The data
processing system 100 further includes a display 120 and an
interface 122. By means of the interface 122, the data processing
system 100 is coupled to the scanner 102 and can also control an
alignment of the component for distorting 106 relative to the
enhanced-density barcode 104.
Such an alignment of the component for distorting 106 relative to
the enhanced-density barcode 104 is used since the enhanced-density
barcode 104 includes a first 116 and a second 118 barcode which are
encoded in the enhanced-density barcode 104. The enhanced-density
barcode 104 includes blocks that are arranged relative to each
other in a geometrical lattice, wherein the component for
distorting 106 is used to distort each block of the set of blocks
of the enhanced-density barcode 104 in a first and a second
direction. These directions are given by the lattice directions of
the geometrical lattice. Each block of the enhanced-density barcode
104 includes four sub-pixels.
For example, the enhanced-density barcode 104 is illuminated by a
light source such that the scanner 102 is able to scan the
enhanced-density barcode 104 through the component for low pass
filtering 108 and the component for distorting 106. After the
enhanced-density barcode 104 has been distorted in a first
direction by means of the component for distorting 106 and has been
low pass filtered by the component 108, the scanner 102 reads the
resulting image. For the first distortion direction, this image is
the first barcode 116 and for the second distortion direction this
image is the second barcode 118. The low pass filtering of the
distorted barcodes results in a color averaging of the sub-pixels
of a block. The reconstruction of the first barcode 116 and the
second barcode 118 includes determining of block boundaries of the
blocks. This is done by, for example, a module 114 included in the
memory 112 which analyzes the image scanned by the scanner 102 and
determines block boundaries such that an accurate reconstruction of
the first and the second barcodes 116 and 118 can be performed. For
example, the block boundaries are determined by the module 114 by
analyzing special alignment patterns included in the image scanned
by the scanner 102.
Even though according to the example as illustrated with reference
to FIG. 1 the distortion component 106 and the low pass filtering
component 108 are analogue optical components like lenses, low pass
filtering and distortion can also be performed within the data
processing system 100. According to such an embodiment of the
invention the scanner 102 is provided with a sufficient high
resolution to resolve the individual sub-pixels of the
enhanced-density barcode 104.
FIG. 2 is a block diagram of a barcode writer. The barcode writer
includes a data processing system 200 which includes a processor
202 and a memory 204. By means of an interface 208, an image
scanner 212 is connected to the data processing system 200. In
operation, the image scanner 212 scans, for example, a first
barcode 116 and a second barcode 118. By means of a module 206, the
first and the second barcode 116 and 118 are encoded into a
two-dimensional enhanced-density barcode which is then written by
means of a writer 210 onto a surface, like an identity card. For
example, the writer 210 may be a laser writer.
FIG. 3 is a schematic illustrating an embodiment of the encoding
and decoding process according to the invention. The first barcode
116 and the second barcode 118 are combined to an enhanced-density
barcode 104. By applying anisotropic filtering, for example by
means of a birefringent lens, a "smeared down" image 300 is
generated. By changing the anisotropy direction of the filtering by
90 degrees, a "smeared right" image 302 is obtained. The smeared
down image 300 and the smeared right image 302 contain areas with
different shadings. In order to convert these shadings to a single
black or white value, a low pass filter is applied to each the
smeared down image 300 and the smeared right image 302. In case of
the smeared down image 300, a barcode 316 is obtained which
corresponds to the originally encoded first barcode 116. The same
holds for the smeared right image 302 which, upon low pass
filtering, is transformed into a barcode 318 which corresponds to
the original second barcode 118.
FIG. 4 is a schematic illustrating a symbol set used to perform an
encoding process. In the following, for the case of simplicity, it
is assumed that the first and the second direction in which the
distortion process is performed are perpendicular to each other.
Further, without loss of generality, it is assumed that input
images have the same dimensions so that pixel grids of input images
can be overlaid exactly. The input images represent the first
barcode 116 and the second barcode 118 that are used as input to
encode them into the enhanced-density barcode 104. In order to
encode the first 116 and the second 118 barcode in the
enhanced-density barcode 104, a pair wise encoding of pixel pairs
of the first 116 and the second 118 barcode is performed according
to a predefined encoding scheme. The predefined encoding scheme
allocates to each combination of pixel pairs a predefined block
comprising a predefined number of sub-pixels. Such a pixel pair
consists of a pixel of the first barcode 116 and a corresponding
pixel of the second barcode 118. Each pixel pair of the first
barcode 116 and the second barcode 118, also denoted as pair of
input pixels, is mapped on a predefined block including a
predefined number of sub-pixels. In this example, each pair of
input pixels is mapped on a block that consists of four sub-pixels.
The sub-pixels are subdivisions of a block. The blocks and the
sub-pixels have in this example the form of a square. By this
mapping process, an array of n.times.m blocks is generated if input
images, i.e. the first barcode 116 and the second barcode 118, are
n pixels wide and m pixels high. Each block corresponds to a pair
of input pixels.
Since each block is defined in terms of four sub-pixels, 2.sup.4
possible variations of four sub-pixels exist. However, according to
a preferred embodiment of the invention only five variations of
these sub-pixels in a block are used for encoding any pair of input
pixels. These blocks are shown as `symbols` 400, 402, 404, 406 and
408.
The distortion of the enhanced-density barcode 104 including the
blocks 400-408 extends dark sub-pixels by a lower intensity, one
sub-pixel wide grey region which intrudes upon neighboring light
pixels in the smearing direction.
For example the `all light block` 408 contains four light or white
sub-pixels, wherein the `all light block` is immune to smearing
from top or from the left. As it may have up to two sub-pixels
turned into grey which result from a smearing of dark sub-pixels
adjacent to the block 408, low-pass filtering always reconstructs
the block 408 into a light pixel. As an `all light block` contains
no dark sub-pixels, it may not influence its right or lower
neighbor during the distortion process.
The contrast to the `all light block` 408 is the `all dark block`
400. The `all dark block` 400 consists of four dark sub-pixels,
which remain unconditionally dark upon filtering. The `all dark
block` 400 is immune to smearing and distortion, as its sub-pixels
may not be further darkened. Nevertheless, the `all dark block` 400
smears grey sub-pixels onto its neighbors in both perpendicular
smearing directions.
The `almost dark block` 402 contains a single light sub-pixel in
the lower right area. The block 402 remains unconditionally dark
during low pass filtering. Nevertheless, it may influence one
sub-pixel of a neighbor it smears onto. Since an `almost dark
block` 402 contains dark pixels in its top and left halves, it is
itself immune to smearing and can obviously not be further
darkened.
The `vertically split block` 404 contains a row of dark sub-pixels
above light ones. With 50% dark sub-pixels, the low pass filter
turns the original block into a light pixel. When smearing down,
the lower row of sub-pixels turns grey, crossing the predefined
threshold and the low pass filter maps the block into a dark
pixel.
The same principle holds for the `horizontally split block` 406.
The `horizontally split block` 406 combines a dark column of
sub-pixels on the left with a light column on the right. This means
that left/right smearing darkens sub-pixels in the right column,
tilting the sub-pixel balance to dark. Top-down smearing preserves
the 50% dark sub-pixel ratio, allowing a low pass filter for
example to restore the complete block to light, depending on the
thresholds of the low pass filter.
The sub-pixel distribution overview 410 illustrates how possible
color distributions of blocks in the symbol set of the symbols
400-408 are either quantized to light or to dark, depending on the
threshold. In the example of the sub-pixel distributions 410, the
low pass filtering has a threshold of 50%, wherein a grey color
value of 50% darkness or below 50% darkness is quantized to light
and above this threshold is quantized to dark.
In FIG. 5, the reference numeral 500 indicates how blocks according
to an encoding scheme of an embodiment of the invention are
selected based on the possible pixel combinations of a pair of
input pixels of the first barcode 116 and the second barcode 118
using the symbols 400-408. The only multiple choice combination is
a dark/dark pair with the symbols 402 and 400.
The reference numeral 502 indicates how left/right distortion
alters the blocks. Sub-pixels in grey are darkened within the block
in question. Separate grey areas, highlighted by crossing, are
sub-pixel impacted outside this block, i.e. within its right
neighbor. Obviously, the two blocks in the top table row remain
light due to retaining two or more light sub-pixels, while the
three in the lower row remain or are turned into dark after low
pass filtering. The block smearing illustration 502 also highlights
the requirement of the `horizontally split block` 406. In order to
preserve its balance, its immediate left neighbor must not smear
and darken its bottom left sub-pixel shown by the arrow. Out of the
five blocks, only an all dark one would do this. Therefore if a
dark/dark combination is needed in the block to the left, it should
be encoded as `almost dark`.
The reference numeral 504 shows the same effect for a top-down
distortion. Grey sub-pixels are highlighted under the same rules
and `out of block` sub-pixels are within the block immediately
below. Two blocks in the left table column remain light, both with
two or more light sub-pixels and the other three blocks remain or
are turned into dark.
FIG. 6 is a schematic illustrating a block encoding and effects of
smearing. With respect to FIG. 6, again the symbols 400-408 are
used to encode two images of the first barcode and the second
barcode respectively into one enhanced-density 2D barcode. Pixels
dark in both images, i.e. dark pixel pairs, are encoded as a
majority dark block. One corner is encoded as light to keep the
`all dark block` from smearing to neighbors. As smearing cannot
lighten, just darken pixels, a majority dark block always gets
filtered to a dark pixel during decoding.
Similarly, light pixel pairs, i.e. pixel pairs consisting of a
light pixel in the first barcode and a light pixel in the second
barcode, are encoded as an `all light block`. Note that an all
light block, also denoted as all white block, may be influenced by
smearing from its immediate neighbors. However, as the darkness
filter threshold requires a majority of dark, an all white block is
always restored to a light pixel after low pass block filtering.
Pixels light after horizontal smearing and dark after top-down
smearing are encoded as a horizontal stripe, two dark sub-pixels
over two light ones.
Horizontal smearing does not change block darkness which stays
light since a majority of dark sub-pixels is needed to turn a block
to dark. Downwards smearing darkens lower, light sub-pixels and
darkens the entire block during low pass filtering. Conversely,
dark/light blocks are encoded as horizontal stripes, smeared
horizontally to dark, and remain light otherwise.
In the case of the symbol 404 it is turned into a light block after
smearing to the right and low pass-filtering. For the case of
smearing down and low pass filtering of the symbol 404, due to the
presence of the black horizontal bar (consisting of two dark
sub-pixels) and the grey horizontal bar, the average color value of
this block is above the threshold, so that in case of smearing down
and subsequent low pass filtering the symbol 404 is turned into a
dark block.
With respect to the symbol 408 in FIG. 6 it is further illustrated
how a neighboring block can influence the symbol 408, for example,
for the smearing to the right it is assumed that left to the symbol
408 a further block is located which in its upper half has a dark
bar of two dark sub-pixels. Due to the smearing to the right, the
dark bar of the neighbor of the symbol 408 influences the symbol
when smeared to the right. This is indicated by the shading.
The same holds for a smear down situation, however, in this case
with a further bar located above the symbol 408. In this case,
smearing down will generate a smearing of the dark bar into the
symbol 408, which is indicated as a shading in the block below the
symbol 408. Nevertheless, in both cases, low pass filtering will
yield a complete light block since the area indicated by the
shading within the smeared symbol 408 is not sufficient to provide
an average color value which is above the threshold for turning the
symbol into a dark block.
Sub-pixel patterns are selected to produce the calculated
anisotropic effect when filtered through simple analogue filters.
In the implementation examples, sub-pixel axes are aligned parallel
to main directions of scanning equipment and anisotropic filtering
in two perpendicular directions (parallel to barcode axis) results
in two noticeably different images.
Note that the proposed sub-pixel resolution barcodes may not be
valid individually, but low pass filtering transforms them into
valid lower resolution barcodes by construction. This means that it
is possible to rely on scanning equipment reconstructing different
binary content reliably from the reconstructed images, using
standard decoding techniques. Decoding assumes, without loss of
generality that the orientation of a 2D enhanced-density barcode
may be ambiguously determined.
Anisotropic image manipulation, called `smearing` which is
spreading of dark sub-pixels across lighter ones, is used to
transform the enhanced-density barcode selectively. The sample
encoding applies left to right and downwards smearing, which
naturally corresponds to the scanning process. The sample sub-pixel
encoding presented above has the advantage that a block appears
dark when smeared in one direction and remains light when smeared
into a perpendicular direction.
* * * * *